linux-kernel - Re: [PATCH 3/6] shm: add memfd

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Message-ID: <CANq1E4SLp5Smxf3VbjT0hq0UrWWDf0RxudbQTA1G=sW1ecQk-g@mail.gmail.com>
Date:	Thu, 20 Mar 2014 12:29:54 +0100
From:	David Herrmann <dh.herrmann@...il.com>
To:	Pavel Emelyanov <xemul@...allels.com>
Cc:	Cyrill Gorcunov <gorcunov@...il.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Hugh Dickins <hughd@...gle.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Karol Lewandowski <k.lewandowsk@...sung.com>,
	Kay Sievers <kay@...y.org>, Daniel Mack <zonque@...il.com>,
	Lennart Poettering <lennart@...ttering.net>,
	Kristian Høgsberg <krh@...planet.net>,
	John Stultz <john.stultz@...aro.org>,
	Greg Kroah-Hartman <greg@...ah.com>, Tejun Heo <tj@...nel.org>,
	Johannes Weiner <hannes@...xchg.org>,
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ryan Lortie <desrt@...rt.ca>,
	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
Subject: Re: [PATCH 3/6] shm: add memfd_create() syscall

Hi

On Thu, Mar 20, 2014 at 10:01 AM, Pavel Emelyanov <xemul@...allels.com> wrote:
> On 03/20/2014 12:47 PM, Cyrill Gorcunov wrote:
>> If I'm not mistaken in something obvious, this looks similar to /proc/pid/map_files
>> feature, Pavel?
>
> It is, but the map_files will work "in the opposite direction" :) In the memfd
> case one first gets an FD, then mmap()s it; in the /proc/pis/map_files case one
> should first mmap() a region, then open it via /proc/self/map_files.
>
> But I don't know whether this matters.

Yes, you can replace memfd_create() so far with:
  p = mmap(NULL, size, ..., MAP_ANON | MAP_SHARED, -1, 0);
  sprintf(path, "/proc/self/map_files/%lx-%lx", p, p + size);
  fd = open(path, O_RDWR);

However, map_files is only enabled with CONFIG_CHECKPOINT_RESTORE, the
/proc/pid/map_files/ directory is root-only (at least I get EPERM if
non-root), it doesn't provide the "name" argument which is very handy
for debugging, it doesn't explicitly support sealing (it requires
MAP_ANON to be backed by shmem) and it's a very weird API for
something this simple.

Thanks
David
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